The present invention pertains to cooling systems and, more particularly, to a fan assembly incorporating blades which may be adjusted to vary the pitch thereof in order to alter the airflow characteristics of the fan assembly. The invention is specifically directed to a control system for use in regulating the blade pitch of such a fan assembly, as well as a method of controlling the pitch of the fan assembly, to develop an optimal airflow based on sensed operating conditions.
Providing a fan assembly including a plurality of circumferentially spaced blades for developing a flow of air for cooling purposes is well known. Such fan assemblies are widely used in numerous fields, such as for cooling heat generating devices. For example, in the automotive art, fan assemblies are commonly used for engine cooling purposes. More specifically, a fan assembly is generally attached to a block of the internal combustion engine and is driven by the engine through a sheave and belt drive arrangement. The fan assembly mainly delivers a flow of air across a radiator and is incorporated as part of an overall, thermostatically controlled engine cooling system.
Since the fan assembly is driven by the engine, the rotating speed of the fan blades tracks the engine's rpm. However, the fan assembly drive typically incorporates a clutching mechanism such that the fan assembly either assumes an off condition, wherein no airflow is generated by the fan assembly, or an on condition, wherein the fan assembly is driven at a maximum rate established by the engine speed. With such an arrangement, a considerable initial load is placed on the drive system, particularly the belts, when the clutching mechanism is activated while the engine is running at a high rate of speed. Another problem associated with such typical engine cooling arrangements is that there is no control over the amount of power the fan assembly will use. Instead, the horsepower draw on the engine is always at a predetermined power versus fan speed relationship, i.e., power draw is cubic in relation to the rotational speed of the fan, while accounting for air density and temperature factors. This is particularly disadvantageous when cooling needs are low, but the fan assembly is still activated at a high speed. Furthermore, engaging the fan assembly can be a major source of noise, especially at low engine rpm. For instance, when the engine is idling, noise generated by the engagement of the fan can be quite disturbing, with the majority of the noise being produced by the frictional engagement of the elements within the clutching mechanism.
Mainly due to the problems outlined above, variable speed fan assemblies, such as those incorporating viscous and eddy current-type fan clutches, and variable pitch fan assemblies have been developed. In general, variable speed fan assemblies are advantageous as the operating speed of the fan blades can be correlated to the degree of cooling required. Of course, variable speed fan assemblies still only provide a set airflow rate at any given fan operating speed. In addition, viscous drives generally cannot provide a fully "off" condition or a "maximum" airflow condition since they are continuously slipping. Here, variable pitch fan assemblies can be advantageously used since the pitch of the blades can be adjusted according to prevailing cooling requirements such that a reduced power draw from the engine can be achieved. Furthermore, variable pitch fan assemblies can be initially engaged in a smooth and quiet manner, even at low engine speeds, and can readily assume both full off and full on conditions.
As indicated above, a major use for the fan assemblies of concern is to produce an airflow used in cooling an engine of a vehicle. In a vehicle environment, it is known for the engine to be linked to a control module which is part of an overall communications network used to supply operational information to many system components of the vehicle. One particular channel commonly found on such a network is a pulse width modulated signal used to inform the engine cooling system of needed cooling requirements. The signal typically has a frequency range of operation considered to act between 0 and 100%, with a 0% signal indicating that no cooling is needed and a signal of 100% representing that a maximum level of cooling is required.
There exist viscous fan assemblies which utilize the pulse width modulated signal from the engine control module (ECM) to vary the amount of slippage permitted in the rotational drive of the fan assembly. In this manner, the slippage can be regulated to vary the degree of cooling provided. In determining the degree of cooling, various factors need to be considered, such as the fan speed and the geometry, diameter, airfoil shape and angle of attack of the blades. Fixed pitch fan assemblies, as in the case of viscous fans, can be driven at different speeds to vary the created airflow, but the fixed characteristics of the blades only enable these types of fan assemblies to operate efficiently in only a small range of speeds.
Therefore, there exists a need for a fan assembly and system for controlling the same which is designed to establish optimal cooling airflow rates in an efficient manner at any speed of the engine.